Non-color-dependent or regular vignetting is a photographic phenomenon in which the picture taken by the camera exhibits a darkening from the center to the corners irrespective of what color is present. Photographs taken by both a film camera and images taken by a digital camera can exhibit vignetting. Some amount of vignetting is acceptable, and actually desirable from an artistic sense. But an excessive amount of vignetting will be objectionable to most people.
In either a film camera or a digital camera, such vignetting is caused by having to capture an image on a small flat surface, namely the film or the optical-to-electronic imager, e.g., a charge coupled device (“CCD”). In either type of camera, light striking the corners of the film/imager travel a longer path and arrive at a different angle of incidence than light which impinges directly upon the center of the film/imager.
Differences in the response of the CCD to light impinging directly versus light impinging at an angle produce non-uniformity in the data corresponding to an image, some of which is characterized as color-dependent vignetting and some of which is characterized as color-dependent vignetting. The lens system can also effect vignetting. Plus, if the camera uses a flash device, vignetting can also be due to the flash device's variation in illumination intensity across the subject.
As the name suggests, color-dependent vignetting is to be understood as a vignetting pattern that differs depending on the particular color of light. A typical color image sensor in a digital camera includes a mosaic type of image sensor, i.e., a CCD over which is formed a filter array that includes the colors red, green and blue. Each pixel has a corresponding red, green and blue filter area. A typical arrangement of the color filter array is the Bayer pattern that uses a repetition rate of four pixels, namely one red pixel, one blue pixel and two green pixels. The larger number of green pixels represents an adaptation for the increased sensitivity to green light exhibited by the human eye. The separate color arrays, or planes, of data images formed by the sensor are then combined to create a full-color image after suitable processing.
Color-dependent vignetting can be exhibited in addition to non-color-dependent vignetting. The human eye is very sensitive to color-dependent variation. Hence, color-dependent vignetting is very objectionable, even in small amounts.
Color-dependent vignetting can be minimized by using higher quality lens systems, image sensors and flash devices. But this is an impractical solution in a situation in which the minimization of system cost and/or system complexity are important design criteria.